The Autonomous Lagrangian Circulation Explorer (ALACE)

The autonomous Lagrangian circulation explorer (ALACE) is a subsurface float that cycles vertically from a depth where it is neutrally buoyant to the surface where it is located by, and relays data to, System Argos satellites. ALACEs are intended to permit exploration of large-scale low-frequency currents and to provide repeated vertical profiles of ocean variables. ALACEs periodically change their buoyancy by pumping hydraulic fluid from an internal reservoir to an external bladder, thereby increasing float volume and buyoancy. Because positioning and data relay are accomplished by satellite, ALACEs are autonomous of acoustic tracking networks and are suitable for global deployment in arrays of any size. While providing only a sequence of displacements between surfacing intervals, ALACEs are efficient in gathering the widely spaced long-term observations needed to map large-scale average flow. The primary technical challenges met in the ALACE design are maximizing energy efficiency to achieve a lifetime of 50 surfacing cycles over several years, achieving reliable satellite communication with minimal surface buoyancy, and developing overall system reliability in an instrument that cannot be recovered or diagnosed after most failures. This paper describes the ALACE system, design specifications, and some field experiences. The singular failure of a simple dynamical model to predict the surface following behavior of scale models in laboratory tests serve as a cautionary note in using simple models to infer the dynamics of surface floats in various oceanographic applications. The limitations of interpreting the sequence of net displacements between surface positions, including errors caused by surface drift, are also discussed.